25 August 2013

Scientists will use cosmic rays to peer inside Fukushima reactor

The news just keeps coming
from Fukushima, questioning both the reported volume and radioactivity
of the water leaking into the ocean, and projections continue to get
less optimistic by the day. The only encouraging part is that the
potential solutions seem to be almost as numerous as the newly reported
problems, as authorities are in the midst of building a steel wall to
contain flowing ground water, and now want to freeze a section of the
perimeter to create a second wall
of icy earth. These are all ultimately stop-gap measures, however,
designed to minimize the continuing impact while cleanup efforts
continue in earnest. The biggest problem for actually improving the
state of the core: cleanup crews know very little about what’s actually
going on inside.

More than six feet of concrete and eight inches
of steel surround the core of the reactors, so getting a picture of the
situation inside has proved extremely difficult. Radiation levels are
obviously too high for human workers, and even pose problems for most
potential robotic solutions. The international community needed to
figure out how to see inside the core without actually going inside,
either personally or remotely, and that meant turning to those few
high-energy particles that could penetrate its thick shielding twice, enter on one side and emerge on the other. After many months of campaigning,
scientists at Los Alamos National Laboratories are proceeding with a
plan to take advantage of one such particle: muons originating from
cosmic rays.

A simple schematic representation of the detector.

In essence, the idea is
similar to many conventional forms of imaging, tracking the path of rays
to walk back to the material that must have caused any observed
deflections. The novel part of the plan is the type and source of the
particle; muons have such high energy that even a detonating nuclear bomb
won’t create them. One readily available source of high-energy muons is
collisions between cosmic protons and atoms in the upper atmosphere.
The pions created in these events quickly decay into muons that are so
penetrative we can detect them at the Soudan II detector more than 700
meters below ground.

The plan uses detectors adapted from border
security machines designed for finding weapons-grade plutonium in cargo
containers. Two detectors will be used, one to measure the original
trajectory before entry, and another to measure the final trajectory
after exit. They’ve already tested the idea on a small-scale mockup of
the reactor and on a working power plant at the University of New
Mexico, so they’re confident in their technique. Researcher Haruo
Miyadera says the plan won’t produce much in the way of scientific
surprises, adding that “all the challenges are in engineering, not in
science.”

The technique is most useful with regard to heavy
materials, and can tell the researchers the locations of large chunks of
fallen building material, as well as the fate of the reactor’s nuclear
fuel, much of which has melted into a concrete vessel below the core.
Water irradiated by this fuel has been leaking to various extents since
shortly after the earthquake of March, 2011.

This test saw the hole in this
lead construct through six meters of concrete, and was conducted in just
over a week; Fukushima Daiichi is much larger, and the detector will
run for months.

Since the source of the
“interrogator” particles is the random collision of particles with the
atmosphere, it will take a long time to resolve a clear picture of
what’s going on inside. Each muon is a data point, but we can’t control
their frequency or trajectory and must wait patiently for enough useful
hits to accrue. The Japanese government hopes to have the detectors set
up in 2015, but doesn’t expect to have a useful picture for several
months after that.

At present, Fukushima Daiichi is in a state
that could be called stable but dangerous. Japanese authorities are
working to contain its only current method of damaging the surrounding
area, the leaks due to cracks in containment. It’s unlikely that the
situation will take a dramatic turn for the worse at this point, and the
team from Los Alamos labs is hopeful its detector can help start a safe
clearing of core debris by 2020. After that, they say, the detector
could become part of the regular nuclear cleanup and maintenance regime.

Despite
being involved in cleaning up this messy situation, LANL scientist
Christopher Morris remains optimistic about nuclear power as a whole. He
reminded NBC News that nobody has yet died as a direct result of the radioactivity at the Fukushima plant.